Does the markings on the sub dial at 3,6,9 ring a bell? Well, it should.
On many vintage chronograph watches from the 50's, a sub dial used for minutes of the chronograph has distinguishable markings on the 3, 6, 9 minutes. Some have speculated on the usage and that it was used for pointing the sun, or used as an instrument in the aviation, naval. But one theory came out to be more agreed in the watch community.
In the 50's, an international call cost one coin in public phone cabins, and allowed a call to last 3 minutes. Once the 3 minute range was passed, the phone call stopped, or if the person had added a coin, the call would last 3 minutes more, and would continue to 6 minutes. As you might have understood, the markers on the watch allowed the person using the phone to control the time spent and more importantly that he would clearly see if the call reaches the 3 minute, 6 minute, or 9 minute marker. Above 9 minutes, the person was probably too rich or either very talkative to care about the time.
You can find here few examples on a vintage watches:
Rolex
Zenith
Berthoud
